The present invention relates generally to MR imaging and, more particularly, to metabolic MR imaging of a hyperpolarized agent.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which frequency is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is generated by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. It is desirable that the imaging process, from data acquisition to reconstruction, be performed as quickly as possible for improved patient comfort and throughput.
For some procedures and investigations, it is also desirable for MR images to display spectral information in addition to spatial information. One known method for acquiring MR signals and reconstructing MR images containing both spatial and spectral information is “chemical shift imaging” (CSI). CSI has been employed to monitor metabolic and other internal processes of patients, including imaging hyperpolarized substances such as 13C labeled contrast agents and metabolites thereof. However, after injection of the hyperpolarized agent, imaging is a challenging task. The hyperpolarization of the agent has a limited lifetime, and imaging must be done rapidly. For example, typical T1 lifetimes of hyperpolarized agents are on the order of a few minutes in vivo. Furthermore, the RF excitations of the pulse sequence may destroy the hyperpolarization irreversibly.
The CSI method has some drawbacks which limit available signal-to-noise ratio, and thus image quality. For example, CSI tends to acquire data slowly, considering the short lifetimes of the increased magnetization of hyperpolarized substances. In addition, CSI typically exposes the imaging subject to a large number of RF excitations. These properties are especially unfavorable for a hyperpolarized agent because the hyperpolarized agent magnetization has a limited lifetime and is destroyed by the RF excitations of the CSI sequence. As a consequence, the available magnetization cannot be fully utilized by the CSI method, and the signal-to-noise ratio (SNR) is thus reduced.
Additionally, MR procedures which require very fast, or periodic data acquisition, such as cardiac imaging or metabolic imaging of the heart, are difficult to perform with CSI sequences because CSI can take more than 15 seconds for a 16×16 matrix, whereas cardiac and related metabolic imaging should be completed within a few heartbeats or a few seconds.
It would therefore be desirable to have a system and method capable of exciting and imaging a metabolic species of a hyperpolarized agent without affecting magnetization of metabolic species at other frequencies.